Take a glance at Capitol Hill right now. Something rare and wondrous is happening.

It is a momentary, precise alignment of the stars and the planets, a sort of legislative solar eclipse.

What is coming together in Congress is a fragile, bipartisan consensus to make some small and overdue reforms to the criminal-justice system, to correct some of the inequities and imbalances that Congress and President Bill Clinton created with draconian anti-crime legislation enacted during the 1990s.

Were the measure to come to the Senate floor during the lame-duck session of Congress, Grassley told me, he believes it would get as many as 70 votes. And if the Senate passes it, House Speaker Paul D. Ryan (R-Wis.) has signaled he could quickly get it through the House, which passed a different version of the bill in May.

“When do you get a chance where, after four years, you compromise and compromise and compromise, and you come up with something that a Republican president, a bipartisan group in the House, a bipartisan group in the Senate, agree on something?” Grassley asked.

Now. Now is that chance.

And that window of opportunity will not be open for long.

Once the House is turned over to Democratic control in January, it no doubt would insist on something more far-reaching. That in turn would never get through the more conservative Senate, which will have two more Republicans than it does now.

The biggest obstacle right now is Senate Majority Leader Mitch McConnell (R-Ky.), who has thus far not committed to bringing the First Step Act to the floor.

His initial, unconvincing argument was that there is not enough time to consider the bill in the less than three weeks of legislating that remain, when the Senate will also be dealing with judicial nominations, year-end funding and foreign policy questions.

But the real reason is the strong opposition from some of his most conservative members, led by Sen. Tom Cotton (R-Ark.), who says the legislation is “a misguided effort to let serious felons out of prison.” The resistance on the right is getting louder and stronger. While the sponsors of the bill say they are open to some changes, they are fearful of losing Democratic votes if they go too far in hopes of bringing a few more Republicans aboard.

The coming days are critical, as evidenced by the fact that the White House dispatched both Vice President Pence and the president’s son-in-law, Jared Kushner, to press for the bill at the Senate Republicans’ weekly private lunch.

On Tuesday, McConnell said he will be taking a nose count among the GOP majority “to see what the consensus is, if there is a consensus in our conference, about not only the substance but the timing of moving forward with that particular piece of legislation.”

A failure by the Senate to act would be an unusual rebuke of the president by a party that has marched behind him in close formation on just about every other issue.

Criminal-justice reform also affords Trump an opportunity to put a compassionate stamp on his tough law-and-order policies. It is no mere coincidence that he has decided to make a strong push for the bill so soon after his party suffered major setbacks in the midterms, effectively reviving what many had thought to be a dead cause.

“I think Trump would like to have it because he’s going into the 2020 election cycle. It’s something positive he could talk about,” Grassley said.

Still, it is also the right thing to do, and a chance for Congress to actually accomplish something that requires working across partisan and ideological lines. Maybe it is too much to hope that this could become a regular practice, but McConnell should bring the bill to a vote.